The story
From earlier uppedy, equivalent to up + -ed + -y; compare blackity, biggity, and hippity (and possibly also persnickety, pernickety). First attested around 1880 in Uncle Remus by Joel Chandler Harris to describe Jack Sparrer (Jack Sparrow), who tattled on Br'er Rabbit (see quotations below). Compare uppish.
Etymology from Wiktionary, CC BY-SA 4.0.